libprocstat: Drop zfs_defs.c hack, including its _KERNEL define

Now that we have a _WANT_ZNODE we can use that instead of defining
_KERNEL, and we're able to move the code back into zfs.c using a real
znode_t pointer.

Whilst here, tidy the includes.

Reviewed by: imp, kib, markj
Differential Revision: https://reviews.freebsd.org/D50721